os: fix is_abs_path function for Windows systems (#14397)

vlib/os/filepath.v

@@ -0,0 +1,86 @@
+module os
+
+// Collection of useful functions for manipulation, validation and analysis of system paths.
+// The following functions handle paths depending on the operating system,
+// therefore results may be different for certain operating systems.
+
+const (
+	fslash = `/`
+	bslash = `\\`
+	dot    = `.`
+)
+
+// is_abs_path returns `true` if the given `path` is absolute.
+pub fn is_abs_path(path string) bool {
+	if path.len == 0 {
+		return false
+	}
+	$if windows {
+		return is_device_path(path) || is_drive_rooted(path) || is_normal_path(path)
+	}
+	return path[0] == os.fslash
+}
+
+// win_volume_len returns the length of the
+// Windows volume/drive from the given `path`.
+fn win_volume_len(path string) int {
+	plen := path.len
+	if plen < 2 {
+		return 0
+	}
+	if has_drive_letter(path) {
+		return 2
+	}
+	// its UNC path / DOS device path?
+	if path.len >= 5 && starts_w_slash_slash(path) && !is_slash(path[2]) {
+		for i := 3; i < plen; i++ {
+			if is_slash(path[i]) {
+				if i + 1 >= plen || is_slash(path[i + 1]) {
+					break
+				}
+				i++
+				for ; i < plen; i++ {
+					if is_slash(path[i]) {
+						return i
+					}
+				}
+				return i
+			}
+		}
+	}
+	return 0
+}
+
+fn is_slash(b u8) bool {
+	$if windows {
+		return b == os.bslash || b == os.fslash
+	}
+	return b == os.fslash
+}
+
+fn is_device_path(path string) bool {
+	return win_volume_len(path) >= 5 && starts_w_slash_slash(path)
+}
+
+fn has_drive_letter(path string) bool {
+	return path.len >= 2 && path[0].is_letter() && path[1] == `:`
+}
+
+fn starts_w_slash_slash(path string) bool {
+	return path.len >= 2 && is_slash(path[0]) && is_slash(path[1])
+}
+
+fn is_drive_rooted(path string) bool {
+	return path.len >= 3 && has_drive_letter(path) && is_slash(path[2])
+}
+
+// is_normal_path returns `true` if the given
+// `path` is NOT a network or Windows device path.
+fn is_normal_path(path string) bool {
+	plen := path.len
+	if plen == 0 {
+		return false
+	}
+	return (plen == 1 && is_slash(path[0])) || (plen >= 2 && is_slash(path[0])
+		&& !is_slash(path[1]))
+}